Magnetron strap ring structure

ABSTRACT

The radial vanes secured to the inside of a cylindrical anode electrode are short-circuited by inner and outer strap rings which are secured to alternate vanes. The intermediate portions of the strap rings between the points at which the strap rings are secured to the vanes are projected outwardly.

BACKGROUND OF THE INVENTION

This invention relates to a magnetron utilized in a microwave oven,defrosting machine or the like, and more particularly a magnetron havingan improved strap ring secured to radial vanes.

As shown in FIGS. 1 and 2, a prior art magnetron comprises a cylindricalanode 1 made of copper, a plurality of copper radial vanes 2 secured tothe inner wall of the anode and a strap ring 3 comprising inner andouter strap rings 3B and 3A respectively connected to alternate vanes bybrazing 4 for short-circuiting interconnected vanes. At the center ofthe anode electrode 1 is positioned a cathode electrode 5 having afilament 5A. The opposite ends of the anode electrode 1 are hermeticallysealed by end plates 6 and 7 to form an evacuated vessel.

When an electric power is applied to the magnetron, the filament 5A isheated to emit thermoelectrons which are accelerated by the electricfield established between the anode and cathode electrodes 1 and 5 tointeract with the magnetic field created by permanent magnets, notshown, whereby the electrons oscillate in an interaction space definedbetween the inner ends of the vanes and the cathode electrode togenerate a high frequency electromagnetic wave. Finally, the electronscollide against the inner ends of the vanes to generate heat which isdissipated by the outer surface of the anode electrode 1 through thevanes. Accordingly, the temperature of the vanes 2 is higher than thatof the anode electrode 1 and the vanes expand inwardly as shown bydotted lines shown in FIG. 3 due to thermal expansion. On the otherhand, as the termperature rises, the strap ring 3 tends to expandoutwardly. However, since the strapping 3 is soldered to the vanes, onlythe portions of the strap ring 3 between the soldered point to the vanescan expand outwardly as shown by dotted lines in FIG. 3. Consequently,large stresses are applied to the joints between the vanes and the strapring. Accordingly, as the power ON-OFF of the magnetron is repeated, theapplication of the stresses is repeated causing rupture of the strapring by fatigue, thereby shortening the life of the magnetron.

SUMMARY OF THE INVENTION

Accordingly, the principal object of this invention is to provide animproved magnetron wherein the fatigue rupture of the strap ring isdecreased and the life is increased.

According to this invention, there is provided a magnetron of the typecomprising a cathode electrode, a cylindrical anode electrodeconcentrically surrounding the cathode electrode, a plurality of radialvanes secured to the inner surface of the anode electrode for definingan interaction space between the inner ends of the vanes and the cathodeelectrode, and concentric inner and outer strap rings which are securedto alternate vanes to short-circuit the same, wherein intermediateportions of strap rings between points at which the strap rings aresecured to the vanes are projected outwardly from circles passingthrough the securing points.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a plan view showing the essential elements of a prior artmagnetron;

FIG. 2 is a longitudinal sectional view taken along a line II--II inFIG. 1;

FIG. 3 is a partial enlarged view useful to explain the thermaldeformation of various elements of the magnetron shown in FIGS. 1 and 2;

FIG. 4 is a plan view showing one embodiment of the strap ring embodyingthe invention;

FIG. 5 is a graph showing the relationship between the amount ofprojection of the strap ring and the stress; and

FIG. 6 is a diagrammatic representation useful to explain the stresscreated in the strap ring due to the thermal deformation of the vane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 4, a strap ring 31 of this invention is characterizedin that an intermediate portion 31b of the strap ring 31 betweenstationary portions 31a at which the strap ring is soldered as shown by4 to the vanes 2 projects outwardly by a length a from a circle passingthrough the stationary portions 31a and having a radius ρ₀. Although inFIG. 4 only an illustration of the inner strap ring 31 is shown, itshould be understood that the outer strap ring, not shown, has the sameconstruction.

When the inner ends of the vanes 2 extend inwardly as shown in FIG. 3due to thermal expansion, the stationary portions 31a of the strap ring31 move to the inside by the same amount as the vanes in the same manneras in a strap ring having the prior art construction. Where theintermediate portion 31b is projected outwardly by a maximum amount aaccording to this invention, it was found by experiment that thevertical stress and the bending stress created in the intermediateportion 31b when the strationary portions 31a move inwardly decrease ascompared with the prior art construction where no projection is providedas will be discussed later with reference to FIG. 5. As a consequence,even when the thermal deformation is repeated as a result of powerON-OFF operations, the fatigue of the strap ring 31 decreases greatly,thus greatly prolonging the life. Although the configuration of theprojection is not limited to any definite shape, where the projectiontakes the form of a sine curve as shown in FIG. 4, its configuration isexpressed by the following equation which represents the variation ofradius ρ with respect to angle θ, ##EQU1## where θ₀ represents an anglebetween adjacent stationary portions 31a of the strap ring.

FIG. 5 shows the measured stresses at the intermediate portion 31b ofthe strap ring 31, in which abscissa represents the ratio of the amountof projection a and the radius ρ₀ (9 mm) of the strap ring at thestationary portions, that is a/ρ₀, whereas ordinate represents thestress. The graph shown in FIG. 5 shows the vertical stress and thebending stress created in the intermediate portion over a range of θ=30°starting from the stationary portion. The vertical stress means thetangential component of the stress P at a point on the intermediateportion whereas the bending stress a stress that forms a moment M, asshown in FIG. 6. FIG. 6 shows the stress which is created when the strapring has deformed as shown. Where the strap ring is preformed to have anintermediate projection, there is no stress at the initial state and thestress is created as shown in FIG. 6 only when the strap ring undergoesthermal deformation.

As can be noted from FIG. 5, the stress decreases as the amount ofprojection a increases, but there is a limited for the decrease in thestress.

Where a/ρ₀ =0.05 to 0.08, the stress decreases to one half the stresscreated in a strap ring provided with no projection. Where ρ₀ =9 mm,then a=0.45 to 0.72=0.6 mm. Where a/ρ₀ is negative, that is, where theintermediate portions project inwardly from the circle having radius ρ₀,the stress increases as compared to a case where no projection isprovided.

As has been described hereinabove, according to this invention, theinfluence upon the inner and outer strap rings caused by thermal stresscan be alleviated, thereby increasing the life of the magnetron.

What is claimed is:
 1. In a magnetron of the type comprising a cathodeelectrode, a cylindrical anode electrode concentrically surrounding thecathode electrode, a plurality of radial vanes secured to the innersurface of said anode electrode for defining an interaction spacebetween the inner ends of said vanes and said cathode electrode, andconcentric inner and outer strap rings which are secured to alternatevanes to short-circuit the same, the improvement wherein intermediateportions of said strap rings between the points at which the strap ringsare secured to said vanes are projected outwardly from circles passingthrough said securing points, and wherein each strap ring lies entirelyin a plane perpendicular to the axis of the cylindrical anode.
 2. Themagnetron according to claim 1 wherein each projection has a formexpressed by an equation, ##EQU2## where ρ represents the distance fromthe axis as a function of the angle θ about such axis, ρ₀ represents theradius of said circle passing through said securing points, a representsthe maximum amount of projection, and θ₀ the angle between the securedpoints and where 0<θ<θ₀.
 3. The magnetron according to claims 1 or 2wherein said strap rings are substantially uniform in a planeperpendicular to the axis of the cylindrical anode.